After 3 days of testing, I managed to extract from the API, each BackupSize, for each BackupFile, for each Backup, for each VM, for each Job in Veeam Enterprise Manager.
That gives me a nice CSV file with the Job name, it's VM count, and it's "used" disk space on our backup storage infrastructure.
I'm now struggling to obtain the same data for the associated BackupCopyJob, which sometimes can treat multiple "classic" Job at a time.
On the Job definition (of type ImmediateBackupCopy), there's no associated item, nor VM, thus no BackupFiles I can extract the "BackupSize" from.
I also tried using the VBR REST API but it doesn't support any other Job type than "Backup" (understand not vCloud or Backup Copy Jobs).
On my endeavour, I already tried the querysvc, which lead me to BackupSessions and eventually taskSession inside. None of the endpoints contained actual disk object I could use in order to calculate the total size used by a Backup Copy Job. Since the repositories we use are shared amongst customers, I can't trust the "Used Space" of said repo.
Any advice would be much appreciated, using VEM or VBR

Thanks !